Generally, a lifting magnet comprises a rotary magnet disposed on a rotational shaft in the center of a housing, and a stationary magnet placed on inner opposite sides of the housing in correspondence with the rotary magnet, so that the housing exerts an attractive magnetic force when the rotary magnet is rotated in a first direction and allows the same magnetic poles of the rotary and stationary magnets to face each other, but the attractive magnetic force is canceled when the rotary magnet is rotated in a second direction and allows the different magnetic poles of the rotary and stationary magnets to face each other. Thus, the lifting magnet is installed in a carrier such as a crane, etc. to lift and carry a heavy steel plate using the magnetic force.
Such a lifting magnet is disclosed in Korean Patent Publication No. 1998-72201.
A conventional lifting magnet comprises a pinion placed in the middle of the rotational shaft to which the rotary magnet is coupled, and a rack provided in an operating unit and engaged with the pinion, wherein the operation unit moves up and down through the middle inside of the housing. Therefore, when the operating unit moves up or down, the rotary magnet is rotated along with the pinion by an angle of 180 degree, so that the lifting magnet is automatically turned on/off to exert and cancel the attractive magnetic force.
Further, the conventional lifting magnet comprises a separate connecting rod placed in the middle of the rotational shaft and allowing the pinion to rotate on its axis against the rotational shaft when the operating unit moves down; and a cylindrical body formed integrally with the pinion and placed on the connecting rod with a bearing. Here, the cylindrical body comprises therein a locking part having a resilient piece; a cam body to be locked to the locking part; and a disc formed integrally with the cam body, which are respectively arranged in consideration of a rotating direction of the rotational shaft. Further, a magnetic pole holding disc having a ratchet shape, together with a stopper, is provided in one end of the rotational shaft.
However, in the conventional lifting magnet, when the lifting magnet is connected to the crane and moves up from the state that the lifting magnet is put on the ground and the attractive magnetic force is set to be turned off, the operating unit first moves up due to the lifting magnet's own weight and at the same time the attractive magnetic force becomes turned on, so that various foreign materials around the lifting magnet are attached to the lifting magnet and therefore the attractive magnetic force becomes weak, thereby making it difficult to smoothly lift and carry the steel plate. Particularly, the attractive magnetic force can be set to be turned on/off only when the operating unit moves up, so that it is difficult for a user to correctly determine the state of the lifting magnet and to carry the steel plate.
Further, in the case where the attractive magnetic force is set to be turned on, because the same magnetic poles of the rotary magnet and the stationary magnet face to each other, a strong repulsive force is exerted between the rotary magnet and the stationary magnet and makes the rotary magnet tend to rotate with respect to the rotational shaft to turn off the attractive magnetic force. In this case, it is impossible to rotate the rotational shaft in the first direction because the pinion is engaged with the rack. However, with regard to the second direction, the free rotation of the rotational shaft is restricted by only a spring force of the stopper, so that the attractive magnetic force is unstably kept, thereby threatening safety of a worker.
Further, the stopper is used for two incompatible purposes of not only smoothly rotating the pinion while the rack moves up but also holding/preventing the pinion from rotating when the attractive magnetic force is set to be turned on, so that it is not easy to secure the operations of the stopper. Also, because the stopper is exposed to the outside, the stopper may be easily released from a locking state or broken due to strong sway or collision with an obstacle while the steel plate is lifted and carried by the lifting magnet installed in the crane, so that the steel plate is likely to be detached from the lifting magnet and fall.
Besides, the magnetic pole of the rotary magnet is eccentrically formed with regard to that of the stationary magnet, so that the attractive magnetic force of the lifting magnet is weakened when the attractive magnetic force is in the on state. Further, the attractive magnetic force is not completely turned off and remains in the lifting magnet even though the attractive magnetic force is in the off state, so that foreign materials as well as the steel plate are attached to the lifting magnet, thereby weakening the attractive magnetic force and causing risk in safety.